Concerns over the health of the fishery in the Fryingpan River in Colorado have led to a call for an assessment of stream health. Certain native wildlife species, because of their biology, can be especially good indicators of habitat condition. American Dippers (Cinclus mexicanus) are aquatic songbirds that inhabit fast-flowing mountain streams in western North America. As top level specialists Dippers are particularly vulnerable to habitat alteration. Dippers integrate a suite of environmental characteristics in selecting breeding habitat to enable reproductive success. These environmental characteristics include the same characteristics that define healthy mountain stream ecosystems. Dippers are dependent on clean, fast-flowing streams with abundant aquatic insect prey and a sustainable riparian tree canopy. Because Dippers require both healthy stream and riparian habitat, alteration to either will likely result in changes to Dipper population abundance, distribution and reproductive success. Because Dipper success is closely tied to these environmental characteristics, Dippers are excellent biological indicators of stream and riparian health. Surveys of American Dippers and their stream and riparian habitat on the Fryingpan River below Ruedi dam were conducted to provide baseline information for future monitoring and evaluation of the condition of the river. Additionally, the stream and riparian habitat assessment can be used in future monitoring efforts to analyze future Dipper abundance and stream condition.
Dippers have evolved as specialists in mountain stream ecosystems, utilizing aquatic macroinvertebrates for food and the complex physical habitat that natural streams provide for foraging, nest sites and protective cover. These specializations have resulted in a bird that is superbly adapted to this rigorous habitat but also in dependency on a narrow range of food resource and habitat characteristics. Dippers select territories based on suitability of nesting habitat (Price and Bock 1983). Sites are chosen to enhance survival and reproductive success. Nest sites are located close to fast water, inaccessible to predators, protected from floods, located on a horizontal ledge or other support, and typically located 2-3 m above water (Bakus 1959).
Factors that limit Dipper success, and consequently population distribution and abundance, include food density, foraging habitat, and availability of nest sites. Where these factors are impaired Dipper populations are reduced. Dippers rarely choose to breed if food density or nest site quality is below a certain threshold (Price and Bock 1983). Diet of the American Dipper consists almost exclusively of a variety of macoinvertebrates and fish. However, breeding Dippers are more selective in their food choice; nestlings are fed a much larger proportion of Ephemeroptera than adults consume in winter (Vickery 1992). Because the presence of “EPT” ((Ephemeroptera-Plecoptera-Trichoptera) macroinvertebrate taxa communities are indicators of good water quality (Ward and Kondratieff, 1992) and because Dippers positively select “EPT” taxa they can also effectively be used as indices of biotic integrity for the bioassessment of streams. (Feck and Hall 2004). Dippers also need riffles to forage in and mid-stream boulders as perches (Tyler and Ormerod 1994). The physical structure of a stream in combination with the hydrologic regime, form a template that enables successful foraging. Alteration to either factor diminishes habitat conditions essential to Dipper foraging success. Nest sites are commonly located on rocky ledges along streams, behind waterfalls, on large rocks in the stream, and under bridges or other man-made structures (Kingery 1996). Four general requirements for nest sites are that they are: "1) close to water, 2) above high water, 3) inaccessible to terrestrial predators, and 4) on a horizontal ledge or crevice for support" (Price and Bock 1983). Finally, riparian vegetative cover stabilizes and provides a protective refuge for Dippers and also provides nest sites. In Colorado, vegetation cover was significantly related to the distribution of dippers during the summer (Price and Bock 1983).
Changes to stream or riparian habitat that alter the physical structure or biological characteristics of Dipper habitat will impact population characteristics and ultimately survivability. Knowledge of the current distribution and abundance of the American Dipper on the Fryingpan River provides a good baseline indicator of stream health. Future Dipper monitoring and habitat assessments can be used to analyze trends and correlations between changes in Dipper abundance and stream condition.